1. Field of the Invention
This invention relates to a Gamma conversion system, and more particularly, relates to a Gamma conversion system utilizing look-up tables to perform Gamma conversion and the related conversion method.
2. Description of the Prior Art
In a display system, the relation between the gray level of a pixel displayed on the panel and its corresponding inputted pixel data is non-linear; the relation can be represented by a parameter, referred to as Gamma. The aforementioned relation can be shown as a function of a non-linear curve. If an inputted pixel data is x, the gray level of the pixel will be xγ, wherein γ is a Gamma exponent. The Gamma exponent of a cathode-ray tube (CRT) which was commonly used is about 2.2. In order to provide a linear relation for further processing between an inputted pixel data and a corresponding reproduced pixel data, the inputted pixel data to the 0.45 power is first calculated. The method above is usually called a Gamma conversion with a Gamma exponent equal to 0.45. With the Gamma conversion that fits the Gamma character of a CRT, the relation between the inputted pixel data and reproduced pixel data displayed on the CRT can be linear.
There are two conventional Gamma conversion methods for generating the gray level corresponding to an inputted pixel data; the first one utilizes digital signal processing performing a gray level function and the second one utilizes a look up table. The first method takes much time in calculation, so the second method is commonly adopted. In the second method, gray levels before conversion and after conversion are previously calculated and stored into a look up table in firmware. When a machine is turned on, the table will be downloaded by a micro-controller unit to a corresponding memory. When Gamma conversion is needed, a gray level before conversion can be regarded as a destination index, and a gray level after conversion can be read out from the table according to the destination index. The utilization of a look-up table takes less time than that taken by digital signal processing.
Referring to FIG. 1, FIG. 1 is a schematic diagram illustrating the system structure of a display system 1. In FIG. 1, an image processing system 10 comprises an inverse Gamma conversion module 12, a signal processing module 14, and a Gamma conversion module 16. Gamma exponent of the image pixel data inputted from the broadcasting system receiving point 11 is 0.45. However, the image processing in the signal processing module 14 must be under linear situation (i.e. Gamma exponent of the image pixel data is 1). Therefore, in the beginning, inverse Gamma conversion module 12 performs inverse Gamma conversion with Gamma exponent equal to 2.2 on the image pixel data, such that the Gamma exponent of the image pixel data after conversion is 1. Afterward, the signal processing module 14 processes the image pixel data. Then, in order to fit the Gamma character of the image pixel data with the panel 18, Gamma conversion module 16 performs a Gamma conversion with Gamma exponent equal to 0.45 on the image pixel data. Finally, the image pixel data is outputted to the panel 18.
In order to provide smooth colors, inverse Gamma conversion module 12 converts the format of the image pixel data as well. In general, image pixel data of the broadcasting system receiving point 11 can be the format of 8-bit or 10-bit. The 8-bit image pixel data is taken as an example for explanation. The conversion, which converts an image pixel data with fewer bits to an image pixel data with more bits (for instance, converting an 8-bit image pixel data to a 12-bit image pixel data) is called color level raise. In order to raise the accuracy of gray levels, color level raise is commonly adopted.
Then, referring to FIG. 2A and FIG. 2B, FIG. 2A is a conversion curve of the inverse Gamma conversion module 12 of FIG. 1. FIG. 2B is a conversion curve of Gamma conversion module 16 of FIG. 1. The conversion curve in FIG. 2A is a case which has 8-bit in, 14-bit out, and Gamma exponent equal to 2.2. The conversion curve in FIG. 2B is a case which has 14-bit in, 8-bit out, and Gamma exponent equal to 0.45. As shown in FIG. 2A and FIG. 2B, lower gray levels of the two kinds of Gamma conversion both has the problems of color banding, hue losing, or gray distortion.
Table 1 is a look-up table showing a Gamma conversion with 8-bit in, 12-bit out, and a Gamma exponent equal to 2.2 according to prior art. Table 1 only shows a part of lower gray levels. The conversion formula utilized in Table 1 is prior art. For example, U.S. Pat. No. 5,196,924 discloses a relative conversion formula, so the formula will not be described here in detail.
TABLE 1inputtedoutputtedvaluevalue00102030405161728293103114125136147158169171118121914
Table 2 is a look-up table showing a Gamma conversion with 12-bit in, 8-bit out, and a Gamma exponent equal to 0.45 according to prior art. The table only shows a part of lower gray levels.
TABLE 2inputtedoutputtedvaluevalue0016283104115126147148159161017111812181319142015201621172218221923
As shown in Table 1, in prior art, when the Gamma conversion with Gamma exponent equal to 2.2 is performed, different inputted values may correspond to the same outputted value. In other words, even if the inputted gray level is changed, the corresponding outputted gray level may not be changed as well. Therefore, gray distortion is generated. As shown in Table 2, in prior art, with the Gamma conversion with Gamma exponent equal to 0.45, when the inputted gray level is 0 and 1, the corresponding outputted gray level is 0 and 6, respectively. That means, even a panel is capable of showing gray levels ranging from 1 to 5, these gray levels are never shown on the panel because of Gamma conversion. The situation of color banding will cause contour situations of the image displayed on the panel.
Relative designers care much about and have tried to overcome the aforesaid problems. Generally, a Gamma conversion with more bits (e.g. 14-bit or 16-bit) is utilized for getting higher accuracy. However, the look-up table with more bits occupies larger memory. Therefore, the invention is provided to improve the disadvantages of prior art.